The function, reliability and performance of microelectronic devices depend on the use of materials and surfaces that have known properties and meet predetermined specifications. Billions of dollars and countless man-hours have been spent developing the systems and processes used to manufacture microelectronics. In order to characterize and optimize these processes and the resulting materials, it is necessary to measure a wide variety of material properties.
One important characteristic of a conducting or semiconducting material is its work function. The work function is the minimum energy required to remove an electron from a material to a noninteracting point above the surface. Herein, we will define work function to take account of and include all factors that influence the electrical potential of a surface, including band bending, interface charges, and charges in or on a dielectric film. In this case, the work function is determined by the electrical potential difference between the Fermi level of electrons inside a conductive or semiconductive material and the electrical potential of a point outside the surface of the top film in a film stack on the material. Using this definition, the work function of a surface varies with the surface potential, and these terms will be used interchangeably to refer to the same property of a surface.
Surface potential is extremely sensitive to surface condition; and surface potential measurements can be used to detect changes in surface condition, such as contamination or electrostatic charging of dielectric films. It is particularly important to measure and control the surface potential of films used in the fabrication of transistor gate stacks. The surface potentials of these surfaces affect important device characteristics such as the threshold voltages of transistors.
One common method of measuring the electrical potential of a surface utilizes a non-contact voltage sensor to measure the electrical potential difference that forms between the sensor probe tip and a surface when the sensor and surface are electrically connected. This voltage is called the Contact Potential Difference (CPD), and it is simply the difference in the surface potentials of the probe tip and surface. If the surface potential of the probe tip is known, then the surface potential of the measured surface can be determined from the measured voltage difference as:Vsurf=Vprobe+VCPD  (1)where Vsurf is the surface potential of the measured surface, Vprobe is the surface potential of the sensor probe tip, and VCPD is the measured CPD.
Non-contact voltage sensors can take many forms. These include vibrating Kelvin probes and Monroe probes. Both Kelvin and Monroe probes utilize a conductive probe tip that is positioned close to the measured surface. The probe tip is electrically connected to the surface, and the probe and the surface form a capacitor. The voltage that forms across this capacitor is equal to the difference in surface potentials between the probe tip and measured surface. This voltage is the Contact Potential Difference mentioned earlier. In order to measure the CPD between the probe tip and the surface, the capacitance between the probe tip and surface is varied. In the case of a Kevin probe, the capacitance is varied by vibrating the probe tip perpendicular to the surface. In the case of a Monroe probe, the capacitance is varied by vibrating a shutter between the probe tip and the surface. When the capacitance between the probe tip and sensor changes, a current is generated into the probe tip. The CPD can be determined by varying the capacitance between the probe tip and surface, monitoring the current into the probe tip, and varying a bias voltage applied to either the probe tip or surface. The bias voltage that results in no current into the probe tip is equal to the negative of the CPD between the probe tip and surface.
The charge on a noncontact voltage sensor is given by the equation for a capacitor:Q=CV  (2)Where Q is the charge on the probe tip, C is the capacitance between the probe tip and the measured surface, and V is the voltage between the probe tip and the surface.
The current, i, into the probe tip is the derivative of the charge on the probe tip and is given by the following formula:
                    i        =                                            ⅆ              Q                                      ⅆ              t                                =                                    C              ⁢                                                ⅆ                  V                                                  ⅆ                  t                                                      +                          V              ⁢                                                ⅆ                  C                                                  ⅆ                  t                                                                                        (        3        )            
The current is the sum of two terms: the CdV/dt term and the VdC/dt term. The C(dV/dt) term results from changes in the voltage between the probe tip and the measured surface, and the V(dC/dt) term results from changes in the capacitance between the probe tip and the measured surface. The CPD and bias voltage between the probe tip and measured surface are assumed to be constant during the measurement, so the dV/dt term is equal to 0. In this case, the current into the probe tip is solely the result of the changing capacitance between the probe tip and surface as follows:
                    i        =                              (                                          V                CPD                            +                              V                Bias                                      )                    ⁢                                    ⅆ              C                                      ⅆ              t                                                          (        4        )            where VCPD is the Contact Potential Difference between the probe tip and measured surface, and VBias is the applied bias voltage. When VBias=−VCPD, the voltage is 0 and the current into the probe tip is also 0.
A noncontact voltage sensor can be used to measure the CPD between the noncontact voltage sensor probe tip and a surface. In order to determine the electrical potential of the surface, it is necessary to know the surface potential of the probe tip. If the surface potential of the probe tip is known, then the potential of a surface can be determined by measuring the CPD between the probe tip and the surface, and then calculating the potential of the surface using Equation 1.
One of the biggest challenges in making accurate and repeatable surface potential measurements using a noncontact voltage sensor is determining the surface potential of the sensor probe tip. One method of determining the probe tip potential is to measure the CPD between the probe tip and some reference surface with a known work function or surface potential, and then calculating the probe tip surface potential using Equation 1. However, it is difficult to create a surface with a known surface potential in air. The work function of a surface is extremely sensitive to surface condition. A gaseous environment can interact with a surface and cause its work function, and hence its surface potential, to change over time. The work function of a surface can be affected by oxidation, adsorption of airborne molecular contaminants such as water vapor, or other chemical reactions. These effects can cause the electrical potential of a surface to vary by 10's or 100's of millivolts over a period ranging from minutes to weeks or longer.
One of the most accurate surface potential reference surfaces is created by an electrochemical half cell. An electrochemical half cell consists of an electrode and an electrolyte solution. The half cell has a characteristic potential that is determined by several factors including the electrode material, the preparation of the electrode, the electrolyte in contact with the electrode, and the electrolyte concentration. Under certain conditions, the half cell electrical potential is highly repeatable and stable. Further, a dilute electrolyte solution in electrical contact with the half cell has a surface potential that is equivalent to the half cell potential, so the surface of this electrolyte solution provides a stable and repeatable surface potential reference. The dilute electrolyte solution provides a clean liquid surface that can be created by emptying and refilling a reservoir with fresh solution. However, liquid half cell references require periodic refilling, so they are relatively difficult to handle and automate in a factory environment. For this reason, liquid half cell references are not well suited to frequent calibrations in an industrial environment.
An alternative to frequently recalibrating the surface potential of a noncontact voltage sensor probe tip using a liquid reference surface would be to determine the surface potential of the probe tip infrequently using a liquid reference surface, and then periodically measuring a stable, solid reference surface to monitor and correct for any change in the probe tip potential. In this case the surface potential of the solid reference surface could be recalibrated infrequently using the liquid half cell reference at the same time the surface potential of the noncontact voltage sensor probe tip is calibrated. If the solid reference surface is known to have a stable surface potential, then any change in measured CPD between the probe tip and stable reference surface after calibration using the liquid reference can be attributed to a change in the surface potential of the probe tip. If the potential of the probe tip and a stable solid reference surface are known at some point in time, and the change in CPD between the probe tip and the solid reference surface since that time is also known, then the current surface potential of the probe tip can be calculated.
Creating a solid reference surface with stable surface potential is a difficult problem. As already mentioned, environmental factors can significantly affect the work function and electrical potential of a surface. One proposed solution is to store reference surfaces in a controlled environment. The controlled environment could include controls for various environmental characteristics such as temperature, humidity and illumination. The controlled environment could also consist of a full or partial vacuum, or could be purged with a specific gas or gasses, such as nitrogen. In this case, the reference surface would normally be removed from the controlled environment for short periods of time to measure the CPD between the reference surface and the noncontact voltage sensor probe tip. Storing the reference surface in a controlled environment may result in a more stable work function, but it suffers from at least two drawbacks. First, the reference surface must be removed from the controlled environment for periodic measurements. Removing the reference surface from the controlled environment could induce changes in the reference surface potential. Second, if a change in CPD between the probe tip and the reference surface is detected, there is still some chance that the change is due to a change in the potential of the reference surface and not due to a change in the potential of the probe tip. There is no independent way of determining which surface has changed.
There is therefore an important need for easily providing an efficient, reliable method and system for surface potential calibration of a non-contact voltage sensor.